This invention relates to a technique for stimulating brain waves and, in particular, to evoking an electrical brain response corresponding to a person's balance function.
In order to form a diagnosis of possible disease, clear and objective information is required. Often such information is difficult or impossible to obtain due to the nature of the disease or to the inability or unwillingness of the patient to cooperate. The latter is particularly true for difficult-to-test patients such as babies, patients unable to communicate, patients unwilling to reveal the full range of their disability, and patients with inconclusive or inconsistent symptoms. This can be overcome with evoked response testing. It uses a technique which includes exposing the patient to a stimulus and measuring the resultant brain signals generated in response to the stimulus. Evoked response testing can be non-invasive (i.e. no foreign matter is introduced into the body) so that the information is obtainable safely. Also, testing can be repeated as often as necessary in obtaining all the information needed to evaluate the course of the disease and/or treatment.
Measurement techniques exist and are well known for detecting signals generated in the brain. Electro-encephalography is now recognized and well established. Tests have been developed using electro-encephalography to, for example, provide an indication whether a sensory stimulus propagates through the nervous system to the brain, reveal specific brain activity in response to a stimulus, distinguish a normal from an abnormal response, and to determine whether a disorder is psychogenic rather than organic.
In all of these measurements, a primary obstacle in getting usable results is the low signal-to-noise ratio of the detected signal. The signals which are typically detected through the scalp are low in amplitude. Moreover, the brain has continual activity which creates a background noise from which it is difficult to distinguish the evoked signals due to a particular stimulus.
To overcome this problem, a technique known as computer averaging was developed. It is disclosed in U.S. Pat. No. 3,087,487. A unit which implements this technique can be purchased from Nicolet Biomedical Division in Madison, Wis. by specifying Model No. CA-1000. Briefly, the subject is exposed to a series of stimuli which can be, for example, visual, auditory, or tactile. The electrical responses to these stimuli are measured by electrodes attached to appropriate regions of the head and body and processed by a computerized electro-encephalograph machine. The detected signals are correlated with the generated stimuli. Since the background brain activity averages out at a steady level, a summation of the evoked responses to a plurality of such stimuli creates a processed signal readily distinguishable from the background noise.
Aside from the difficulty of distinguishing the brain's evoked response from its background noise, which has now been overcome, no safe technique has been developed to focus on testing the balance function of human beings with an evoked response. Such information is required, for example, to determine whether the balance mechanism in the ear functions properly and whether the neural pathways are intact and operational. The problem is how to stimulate that particular brain response so a signal can be evoked indicative of the state of the subject's balance function. The known auditory, visual and tactile stimuli used to date in evoked response testing do not evoke the desired balance function brain response.
One technique now in use on human beings for lack of any other to accomplish this aim is known as the "bithermal caloric test". It is a test which involves irrigating the outer ear canal with water. However, it causes side effects such as dizziness and nausea. Moreover, it can stimulate the vagus nerve which controls breathing, and in rare cases death can result if the subject's breathing is thereby impaired. Because of the discomfort and danger involved, it is used relatively sparingly in the first place, and only rarely for repeat testing.
Another approach for evoking a balance function brain response has been used on small animals such as mice and cats. It has been reported in the article "Short Latency Vestibular Evoked Response to Acceleration Stimuli Recorded by Skin Electrodes" authored by Elidan, Sohmer, Lev and Gay appearing in Ann Otol Rhinol Laryngol 93:84, and the article "Recording of Short Latency Vestibular Evoked Potentials to Acceleration in Rats by Means of Skin Electrodes" authored by Elidan, Sohmer and Nizan appearing in Electroencephalograph Clin Neurophysical 1982:53:501-15. Their approach requires rotation of the entire animal within a cage. This is not practical, and may even be dangerous, for human beings.